The heads of major U.N. organizations are calling for access to children and families cut off from humanitarian aid in Syria, saying the world "must not stand silent" even though the high-profile siege of Aleppo is over.

Top officials from the U.N.'s refugee agency, humanitarian aid coordinator OCHA, children's agency UNICEF, the World Food Program and the World Health Organization are urging for "immediate, unconditional and safe access" to those civilians.

They say that up to 700,000 people — nearly half of them children — live in 15 besieged areas of Syria. Many lack "the most basic elements to sustain their lives" and face "continued risk of violence."

The appeal came on Monday from Davos in Switzerland, where business, political and other elites are gathering for the World Economic Forum.